(E)-.beta.-farnesene (FIG. 1) is an acyclic sesquiterpene olefin that occurs in a wide range of both plant and animal taxa. Over 600 papers have been published on the occurrence of this natural product and its deployment as an important courier in chemical communication. The olefin is found in the essential oil of hundreds of species of both gymnosperms, such as Torreya taxifolia (Florida torreya) (Shu, C. K., Lawrence, B. M. and Croom, E. M., Jr. (1995) J. Essent. Oil Res. 7, 71-72) and Larix leptolepis (larch) (Nabeta, K., Ara, Y., Aoki, Y. and Miyake, M. (1990) J. Nat. Prod. 53, 1241-1248), and angiosperms, such as Robinia pseudoacacia (black locust) (Kamden, D. P., Gruber, K., Barkman, L. and Gage, D. A. (1994) J. Essent. Oil Res. 6, 199-200), Medicago sativa (alfalfa) (Kamm, J. A. and Buttery, R. G. (1983) Entomol. Exp. Appl. 33, 129-134), Chamomilla recutita (chamomile) (Matos, P. J. A., Machiado, M. I. L., Alencar, J. W. and Craveiro, A. A. (1993) J. Essent. Oil Res. 5, 337-339), Vitis vinifera (grapes) (Buchbauer, G., Jirovetz, L., Wasicky, M. and Nikiforov, A. (1994) J. Essent. Oil Res. 6, 311-314), Cannabis sativa (hemp) (Lemberkovics, E., Veszki, P., Verzar-Petri, G. and Trka, A. (1981) Sci. Pharm. 49, 401-408), Zea mays (corn) (Turlings, T. C. J., Tumlinson, J. H., Heath, R. R., Proveaux, A. T. and Doolittle, R. E. (1991) J. Chem. Ecol. 17, 2235-2251), Piper nigrum (black pepper), Daucus carota (carrot), and Mentha x piperita (peppermint) (Lawrence, B. M. (1972) Ann. Acad. Bras. Cienc. 44, (suppl.), 191-197).
While socially dominant male mice produce both .alpha.-farnesene and (E)-.beta.-farnesene in their urine as pheromones (Novotny, M., Harvey, S. and Jemiolo, B. (1990) Experientia 46, 109-113), it is in the insects and plants that the use of (E)-.beta.-farnesene as a semiochemical is most extensive. (E)-.beta.-Farnesene is emitted by the Dufour's gland of andrenid bees (Fernandes, A., Duffield, R. M., Wheeler, J. W. and LaBerge, W. E. (1981) J. Chem. Ecol. 7, 453-460) and by several genera of ants (Ali, M. F., Morgan, E. D., Attygalle, A. B. and Billen, J. P. J. (1987) Z. Naturforsch. 42, 955-960; Jackson, B. D., Morgan, E. D. and Billen, J. P. J. (1990) Naturwiss. 77, 187-188; Ollet, D. G., Morgan, E. D., Attygalle, A. B. and Billen, J. P. J. (1987) Z. Naturforsch. 42, 141-146), where it serves both as a defensive allomone and as a trail pheromone. This sesquiterpene is synthesized de novo in the osmetrial glands of larval Papilio (Lepidoptera:Papilionidae) as an allomone (Honda, K. (1990) Insect Biochem. 20, 245-250), and it functions as a feeding stimulant to the sand fly Lutzomyia longipalpis (Diptera:Psychodidae), an important vector of the blood disease leishmaniasis (Tesh, R. B., Guzman, H. and Wilson, M. (1992) J. Med. Entomol. 29, 226-231). Several species of predatory carabid beetles use E-.beta.-farnesene as a prey-finding kairomone (Kielty, J. P., Allen-Williams, L. J., Underwood, N. and Eastwood, E. A. (1996) J. Insect Behav. 9, 237-250). When released by corn, this olefin is also a kairomonal oviposition stimulant to the European corn borer (Ostrinia) (Binder, B. F., Robbins, J. C. and Wilson, R. L. (1995) J. Chem. Ecol. 21, 1315-1327). (E)-.beta.-farnesene is the major component of pollen odor in Lupinus and stimulates pollination behavior in bumblebees (Dobson, H. E. M., Groth, I. and Bergstroem, G. (1996) Am. J. Bot. 83, 877-885). Feeding by larval lepidopterans, such as Heliothis or Spodoptera (Noctuidae), increases the amount of (E)-.beta.-farnesene released by corn; the volatile olefin is then detected as a synomone by the parasitic wasp Cotesia marginiventris (Hymenoptera:Braconidae) for locating the lepidopteran hosts (Turlings, T. C. J., Tumlinson, J. H., Heath, R. R., Proveaux, A. T. and Doolittle, R. E. (1991) J. Chem. Ecol. 17, 2235-2251). Circumstantial evidence also suggests the lepidopteran induced production and emission of (E)-.beta.-farnesene from corn serves as a synomone for Cotesia kariyai (Takabayashi, J., Takahashi, S., Dicke, M. and Posthumus, M. A. (1995) J. Chem. Ecol. 21, 273-287) and from cotton leaves as a synomone for C. marginiventris (Pare, P. W. and Tumlinson, J. H. (1997) Nature 385, 30-31; Loughrin, J. H., Manukian, A., Heath, R. R., Turlings, T. C. J. and Turnlinson, J. H. (1994) Proc. Natl. Acad. Sci. USA 91, 11836-11840).
Perhaps of greatest significance in plant-insect interactions is the use of E-.beta.-farnesene by most aphid species as an alarm pheromone (Bowers, W. S., Nault, L. R., Webb, R. E. and Dutky, S. R. (1972) Science 177, 1121-1122; Edwards, L. J., Siddall, J. B., Dunham, L. L., Uden, P. and Kislow, C. J. (1973) Nature 241, 126-127). Aphids exposed to (E)-.beta.-farnesene become agitated and disperse from their host plant (Wohlers, P. (1981) Z Angew. Entomol. 92, 329-336). Alate aphids are usually more sensitive than are apterae species and will often not colonize a host displaying (E)-.beta.-farnesene. Ants that defend aphids are sensitive to host-emitted (E)-.beta.-farnesene and, when exposed, will display aggressive behavior (Nault, L. R. and Montgomery, M. E. (1976) Science 192, 1349-1351). (E)-.beta.-farnesene also mimics the action of juvenile hormone III in some insects (Mauchamp, B. and Pickett, J. J. (1987) Agronomie 7, 523-529), may play a role in control of aphid morphological types, and is acutely toxic to aphids at a dose of 100 ng/aphid (van Oosten, A. M., Gut, J., Harrewijn, P. and Piron, P. G. M. (1990) Acta Phytopathol Entomol. Hung. 25, 331-342). (E)-.beta.-farnesene vapor is also toxic to whiteflies (Klijnstra, K. W., Corts, K. A. and van Oosten, A. M. (1992) Meded. Fac. Landbouwwet 57, 485-491).
Efforts to control aphid behavior by topical application of (E)-.beta.-farnesene to crops have met with little success, due to volatility and rapid oxidative inactivation in air (Dawson, G. W., Griffiths, D. C., Pickett, J. A., Plumb, R. T., Woodcock, C. M. and Zhang, Z. N. (1988) Pest. Sci. 22, 17-30). Derivatives of (E)-.beta.-farnesene with reduced volatility, or increased stability, have shown promise in reducing aphid-transmitted viruses, such as barley mosaic virus (Dawson, G. W., Griffiths, D. C., Pickett, J. A., Plumb, R. T., Woodcock, C. M. and Zhang, Z. N. (1988) Pest. Sci. 22, 17-30), potato virus Y (Gibson, R. W., Pickett, J. A., Dawson, G. W., Rice, A. D. and Stribley, M. F. (1984) Ann. Appl. Entomol. 104, 203-209), and beet mosaic virus (Gibson, R. W., Pickett, J. A., Dawson, G. W., Rice, A. D. and Stribley, M. F. (1984) Ann. Appl. Entomol. 104, 203-209). The wild potato Solanum berthaultii, which produces (E)-.beta.-farnesene in type A trichomes, is more repellent to the green peach aphid than are commercial varieties of S. tuberosum that produce lower levels of the olefin (Gibson, R. W. and Pickett, J. A. (1983) Nature 302, 608-609; Ave, D. A., Gregory, P. and Tingey, W. M. (1987) Entomol. Exp. App. 44, 131-138). In alfalfa, repellency to the blue alfalfa aphid and the pea aphid is correlated with the leaf content of (E)-.beta.-farnesene, but not with the amount of the co-occurring sesquiterpene caryophyllene (Mostafavi, R., Henning, J. A., Gardea-Torresday, J. and Ray, I. M. (1996) J. Chem. Ecol. 22, 1629-1638).
For plants that produce (E)-.beta.-farnesene, breeding for increased production has met with some success (Mostafavi, R., Henning, J. A., Gardea-Torresday, J. and Ray, I. M. (1996) J. Chem. Ecol. 22, 1629-1638), but has been limited by genetic variation in these species. (E)-.beta.-farnesene synthase has been purified from maritime pine (Pinus pinaster) and characterized (Salin, F., Pauly, G., Charon, J. and Gleizes, M. (1995) J. Plant Phys. 146, 203-209), but the gene has not yet been isolated from any source. A cDNA clone for (E)-.beta.-farnesene synthase would, by transgenic manipulation, provide a valuable addition to the arsenal of natural compounds active in host plant resistance. The substrate for (E)-.beta.-farnesene synthase is farnesyl diphosphate, a ubiquitous isoprenoid intermediate involved in cytoplasmic phytosterol biosynthesis. Sesquiterpene synthases lack plastidial targeting sequences and are localized to the cytoplasm (Chappell, J. (1995) Annu. Rev. Plant Physiol. Plant Mol. Biol. 46, 521-547). Therefore, even in plants that do not normally produce sesquiterpenes, a recombinant (E)-.beta.-farnesene synthase would be directed to the cytoplasm where substrate is supplied by the mevalonate pathway and where production of (E)-.beta.-farnesene should result.